The field of the invention is synthetic resins containing mixtures of two or more solid polyamides. The invention is particularly concerned with the use of binary copolyamides as hot-melt adhesives for articles which come into contact with fuels for internal combustion engines. In particular, the adhesives are intended to achieve a particularly good resistance to lower alcohols, especially ethanol, methanol and isopropanol, which are becoming more and more important as fuel additives and as fuels alone for engines.
The state of the art of hot-melt adhesives may be ascertained by reference to U.S. Pat. Nos. 4,101,534 and 4,196,108; West German Published Application No. 24 44 560 and the Kirk-Othmer, "Encyclopedia of Chemical Technology", 3rd Edition, Vol. 1 (1978) under the section "Adhesives", pp. 488-510, particularly p. 499, where it is disclosed that "Hot-melt adhesive applicators are really miniature thermostatically controlled extruders similar to those used to extrude thermoplastics", p. 503, where the automotive uses are disclosed, p. 506 where plastics joining is disclosed, and pp. 506-507 where the energy limitations of hot-melt adhesives are disclosed, the disclosures of which are incorporated herein.
Hot-melt adhesives of the prior art, for example those based on ethylene/vinyl acetate copolymers (EVA), copolyesters or copolyamides are unsuitable for adhesive bonds on parts which come into contact with engine fuels because of their low resistance to the fuels.
U.S. Pat. No. 4,101,534 discloses a metal-bonding adhesive composition consisting of 95-60% by weight, based on the total composition, of an aliphatic homopolyamide and 5-40% by weight, based on the total composition, of an aliphatic copolyamide, the homopolyamide being a crystalline homopolyamide having up to 14 amide groups per 100 carbon atoms, and the copolyamide consisting of 90-70% by weight of (1) a polyamide having up to 14 amide groups per 100 carbon atoms and 10-30% by weight of (2) at least one polyamide whose recurring units are different from those of the polyamide (1).
According to U.S. Pat. No. 4,196,108, a polyamide resin useful as a hot melt adhesive consisting of recurring units is obtained by reacting in an inert atmosphere at a temperature of 180.degree.-300.degree. C. while removing water of reaction:
(a) 10-50% by weight, based on the total composition, caprolactam;
(b) 10-50% by weight, based on the total composition, of a polyamide or polyamide salt derived from hexamethylene diamine and an alpha-omega dicarboxylic acid of the formula HOOC--R.sub.1 --COOH where R is a C.sub.4 -C.sub.7 bivalent aliphatic hydrocarbon radical;
(c) 10-50% by weight, based on the total composition, of a polyamide or polyamide salt derived from nonamethylene diamine and an alpha-omega dicarboxylic acid of the formula HOOC--R.sub.1 --COOH where R.sub.1 is C.sub.7 -C.sub.10 bivalent aliphatic hydrocarbon radical; and
(d) an amount up to 50% by weight, based on the total composition, of a polymeric fatty acid and alpha-omega aliphatic diamine of the formula H.sub.2 N--R.sub.2 --NH.sub.2 where R.sub.2 is a C.sub.5 -C.sub.7 alkylene radical, a polyamide or polyamide salt derived from the polymeric fatty acid and diamine.
The use of an engine fuel-resistant hot-melt adhesive which is based on binary copolyamides formed from caprolactam and lauryllactam is known from West German Published Application No. 24 44 560. The weight ratio disclosed for caprolactam:lauryllactam is between 60:40 and 30:70, especially 50:50. However, these copolyamides are only suitable for the production of fuel-resistant adhesive bonds if they are extracted with an alcohol or a halogenated hydrocarbon before being used.
Extracting hot-melt adhesives at the granule stage is a very expensive step in the production process. First, this is because of the additional outlay of labor, and second, because of the costs of the extraction agent and its redistillation. Above all, however, the cost is high because of the high loss in material, which in the case of ethanol, can be up to 50 percent by weight for the copolyamides described in West German Published Application No. 24 44 560. The granules swell considerably during extraction and this swelling leads to fractures and finally to the disintegration of the granules to give fine grit. The conveyance of such irregular grit, which contains considerable fine dust, in the screw metering extrusion apparatus used for applying hot-melt adhesives is, of course, not free from problems.
The high solubility and the considerable fracture due to swelling of the prior art binary hot-melt adhesives formed from lauryllactam and caprolactam make it evident that the extraction step is necessary, otherwise the adhesive joint disintegrates when in contact with the polar solvent content in fuels, and as a result of the loss of material there are fractures due to the swelling.